Don't check MX records for autoconfig

Expected behavior

It should query the autoconfig server for the domain of the email address.

Actual behavior

The MX record of the domain is queries and then autoconfig.MX is queried which is wrong.
The MX record is only for other mail SERVERS. It has absolute no meaning for mail clients. The server for mail clients can be a total different server in a different domain. A mail client should not care at all about the MX record.
The correct address would be autoconfig.domain. See https://wiki.mozilla.org/Thunderbird:Autoconfiguration
The actual implementation might work for some bogous setups. I have here especially Cpanel administrated domians in mind. In most setups Cpanel is setting the MX record to the domain name which is wrong. According to the SMTP RFC the MX record has to be a full qualified hostname and not a domain name.

Mail app

Mail app version: 0.18.1

Mailserver or service: Own setup with postfix and dovecot spread over 4 servers and 3 domains.

Server configuration

Operating system: Ubuntu 16.04

Web server: Apache 2.4

Database: MariaDB

PHP version: 7.2

Nextcloud Version: 17.0.1

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Start by tracing the mail autoconfiguration lookup and the code that queries the domain's MX record. Confirm how the email domain is converted into the autoconfig server address. Done means the client queries autoconfig.domain directly without first querying the domain's MX record.